In medical facilities like clinics and hospitals, a variety of modalities, such as devices for CR (computed radiography), CT (computed tomography), MRI (magnetic resonance imaging), PET (positron emission tomography) and ultrasonic diagnosis, have been widely used. Medical images shot by the modalities are used in medical diagnoses, taking an important roll when doctors determine the symptoms of patients.
The medical images are stored in the medical facilities for several years since it is obliged to store them for a certain period of time or they need to be referred for a follow-up. The medical facilities therefore need to store a large amount of medical images. If each medical image is stored in the form of film, a storage space becomes huge, and management and search thereof becomes a heavy burden. To avoid such situation, for example, a diagnosis support system of Japanese Patent Laid-Open Publication No. 2002-282247 digitizes medical images and stores them in a sever and the like. According to this system, the medical facilities need only to prepare the space for the server, and thus the storage space for the medical images can be significantly reduced. In, addition, when the management and the search of the medical images are performed by the server, work performance in the medical facilities can be improved.
By the way, since the stored medical images cannot easily be deleted, a server of such medical system needs to expand its recording capacity when it becomes full. However, the expansion of storage capacity costs a lot. In view of this, compression is often performed to each medical image and thereby reducing consumption of the recording capacity in the server (for example, see Japanese Patent Laid-Open Publication No. 2002-282247).
For the compression, there are lossless compression and lossy compression. In the lossless compression, the image quality is not lost, however it is difficult to drastically reduce the consumption of the recording capacity since the compression ratio is low. Meanwhile, in the lossy compression, although the compression ratio is high, it also involves the image quality loss, and therefore it may cause misdiagnosis.
In view of this, for the lossy compression, a server which effectively reduces the consumption of the recording capacity, while maintaining the image quality, by setting an appropriate compression ratio for each site, such as head or chest, is developed. (For example, see “monthly INNERVISION”, published by Kabushiki Kaisha Innervision, April 2005, extra edition, Volume 20, Number 5 (Consecutive Number 229), p. 182-187). This server uses the theory that the degree of image quality loss varies depending on the site being shot. For example, an image precisely showing a picture of air area and bone area (or blood vessels and bronchial tubes) has high contrast, and therefore the image quality loss becomes distinctive when the compression ratio is high. Meanwhile, an image of neck or abdomen has few area of high contrast, and therefore the image quality loss becomes less distinctive even when the compression ratio is high. Accordingly, low compression ratio is applied for the head or chest, whereas high compression ratio is applied for the neck and abdomen, and thereby the consumption of the recording capacity can effectively be reduced while maintaining the constant image quality.
Medical images shot with the CT scanner or the MRI scanner are slice images capturing cross section of a human body. When using the CT scanner or the MRI scanner, a plurality of slice images shot from different positions are normally obtained in a single examination. In addition, it has recently been possible to shoot across all sites which may be objected for examination at one time owing to improvement in shooting capability of each modality. A group of the shot slice images is classified by site and then stored or transferred to each specialist.
In the event the slice images classified by site are interpreted, some of the adjacent slice images also need to be observed if a lesion is found near the boundary of the sites. In the case that the contrast ratio has been changed from site to site as described above, however, the image quality may drastically change between the adjacent sites like for example the chest and the abdomen, which causes difficulty in the interpretation. Especially when the lesion resigns across the adjacent sites, it is hard to distinguish between the change due to the lesion and the change due to the image quality loss, and it may cause misdiagnosis.
It is an object of the present invention to provide an image compression method, an image compression device, and a medical network system capable of effectively reducing consumption of recording capacity while keeping image quality and also preventing image quality from changing between adjacent sites when storing a plurality of medical images shot in a single examination.